1. Field of the Invention
The present invention relates to improved ceramic compositions which include light curing resins as a binder and to methods employing such compositions, particularly in the field of restorative dentistry.
2. Description of the Related Art
In recent years, the emphasis in restorative dentistry has been toward aesthetics. Metal restorations, including fillings, inlays, onlays, crowns and bridges, are being replaced, where indicated, by more aesthetic materials. These materials are mainly resins and porcelains. Both materials may be used in restorations as a veneer or covering of metal substructures. For even more aesthetic results, restorations are being fabricated entirely of resin or porcelain. Perhaps the ultimate in cosmetic dentistry is the all resin or porcelain laminate. Discolored tooth surfaces are laminated with a thin layer of these aesthetic materials.
Both resin and porcelain are highly aesthetic materials. However, each has its own advantages and disadvantages when being considered for use in restorations. The greatest advantage that porcelain has over resins is its hardness. This property gives porcelain a greater wear resistance. Porcelain's greatest disadvantage is the difficulty encountered during the fabrication of restorations, especially those composed of all porcelain.
Dental porcelain is a fine powder of "glass-like" particles. In order to fabricate a dental restoration, water or some suitable liquid is added to the powder. A wet, sandy mix is created which can be formed into desired shapes. The porcelain particles are fused together by heat producing a solid substance similar to glass. In this manner, porcelain may be enameled to metal or simply baked into a solid mass of pure porcelain. Restorations are usually fabricated on a replica or die of the prepared tooth. Also, materials may be added to the porcelain powders which improve color and strength.
There are two properties of porcelain which complicate the fabrication of the all ceramic restoration. One is the fragile nature of the porcelain-liquid mix after shaping and prior to baking. The other complicating property is the tendency of porcelain to shrink during baking or firing. These complications can be partially overcome but require special materials and time consuming procedures.
One method of making an all ceramic restoration is known as the direct lift technique. Basically, the porcelain-liquid mix is packed and shaped on the die, replacing the missing areas. Next, the dried porcelain is lifted from the die and baked in a furnace. The particles of porcelain are thereby fused together forming the solid restoration. However, both complications mentioned previously become a factor. The dried porcelain is extremely fragile and cannot be lifted from the die without fracturing or being crushed. Secondly, the shrinkage which occurs during baking causes the restoration to distort and not properly refit on the die. These problems can be partially solved but require special materials and extra valuable time.
One such direct lift technique employs a metal foil. In such techniques, a metal foil, usually platinum or gold, is adapted to the area of the die containing the restoration. The porcelain-liquid mix is applied to the foil. After proper condensation and drying of the porcelain, the foil, supporting the porcelain, is lifted from the die and placed in the furnace for baking. After baking, the foil and porcelain restoration is returned to the die. Usually, shrinkage occurs and additional porcelain must be added and the material refired. After the proper shape and contours have been achieved, the metal foil may be removed and the all porcelain restoration is ready for placement in mouth by cementation or bonding.
Restorations using this method include inlays, onlays, crowns and laminates. This technique is also used in the fabrication of porcelain margins on crowns made by fusing porcelain to metal. This method is very difficult where very complicated tooth preparations are necessary.
Another direct lift technique is referred to as the wax additive technique. This technique is designed to make the lift off procedure possible without the use of metal foils. Powdered wax is mixed with porcelain powders instead of the water or special liquid. The die is lubricated with a separator which prevents the sticking of hot wax. The porcelain-wax mix is melted and placed on the die. After cooling, the procelain-wax is shaped by carving. It is removed and baked.
Because of extreme shrinkage and porosity caused by the eliminated wax, this method is only useful in the construction of all porcelain margins or porcelain fused to metal restorations.
A third technique, the porcelain-silica mix technique, employs the use of a liquid containing colloidal silica which is mixed with the porcelain powder. The silica causes the entire mix to harden slightly, allowing it to be lifted from a lubricated die. As with the wax additive technique, this method is generally limited to porcelain margins.
In addition to direct lift techniques, a second method used in the fabrication of all porcelain restorations is known as the refractory cast technique. This procedure involves the use of a special die material that is designed to withstand high temperatures. A die of this material is used in conjunction with the regular die. The porcelain-liquid mix is placed on the refractory die, shaped and fired without being lifted. Additional porcelain additions and refirings are usually necessary in order to compensate for shrinkage. After the desired results are achieved, the restoration is broken away from the refractory die and placed on the master die for a final check. If all is well, the restoration is ready for the mouth. If not, the whole restoration is reconstructed, starting from a new refractory die. This technique is very successful and popular but expensive. All types of restorations may be made from this technique. The construction of the refractory die is not particularly difficult but is time consuming and expensive.
The most recent method of producing all porcelain restorations is called the castable ceramics method. This technique is initiated by creating the restoration in wax. The wax pattern is lifted from the die and invested or surrounded by a mix of "plaster-like" material which is allowed to harden. A channel or opening leads from the outer surface of the investment into the wax pattern. The invested pattern is placed in an oven where the wax is eliminated through the opening. A special casting machine melts a mass of ceramic material and casts it into the opening of the investment. This glass material fills the void created by the wax pattern. After cooling, the hardened ceramic is broken out of the investment. A special oven heats the glass restoration and changes it to a more suitable ceramic form, maintaining the shape. Color is baked onto the surface of the restoration to simulate tooth color. All types of restorations are made in this manner. The fit and aesthetics are excellent. The disadvantages are the time required for fabrication and the high cost of equipment.
Thus, it is apparent that while all ceramic restorations constructed by the foregoing methods are excellent from an aesthetic standpoint, their fabrication has numerous disadvantages and limitations. Accordingly, their is a great need for improved porcelain materials and/or fabrication methods, which will allow the fabrication of ceramic objects in a manner that eliminates many of such disadvantages while allowing uses previously not possible.